Conventionally, there have been cellular systems including user terminals and a base station for performing wireless packet communications with user terminals in its own cell using a shared channel.
In a cellular system, user throughput presents a different value depending on the geographical features around the base station. User throughput is a value that is measured at a user terminal, and is represented by the number of bits of packets that are received by the user terminal from the base station per unit time.
As a method for estimating the geographical distribution of user throughput in a cellular system, it is a common practice that a computer is used to perform a system level simulation of the cellular system for a continuous period of time. The reason is that, because a shared channel is used in common by user terminals in a cell under high-speed control, it is impossible to obtain correct estimation of user throughput unless the shared process is simulated at a high temporal resolution over a continuous period of time while performing a detailed simulation of the process of wireless packet communications including upper layers.
A conventional method of estimating the geographical distribution of user throughput will be described with reference to FIG. 1.
As shown in FIG. 1, a system level simulation of a cellular system over a continuous period of time is performed by system level simulator 300.
System level simulator 300 receives as its input the base station configuration, the amount of traffic of packets that are generated when each user terminal makes a download request, the total number of users in the cell. Here, the base station configuration means the settings and status of the base station, including, for example, the position of the base station, transmission power, antenna pattern, antenna azimuth, antenna tilt, the type of the packet scheduler for scheduling the priority order of packet transmission processes to user terminals and the like.
System level simulator 300, based on the aforementioned input values, estimates the reception quality of the shared channel at the location of each user terminal, and performs continuous simulation of a wireless packet communications process that is close to reality including protocol upper layers, to thereby estimate user throughput. Here, as the reception quality of the shared channel, reception SIR (Signal to Interference power Ratio) which represents the ratio of the shared channel reception signal power to the interference signal power is usually used.
Next, the procedure of a system level simulation over a continuous period of time by system level simulator 300 will be described in detail.
System level simulator 300 simulates the following first to fifth steps when it uses the HSDPA (High SPEED Downlink Packet Access) scheme for high speed downlink access in W-CDMA (Wide band-Code Division Multiple Access) as a wireless packet communication scheme for the downlink from the base station to user terminals.
The first step: user terminals are generated at various positions. Then, the user terminals are caused to make traffic requests to the base station for transmitting packets through the shared channel. Here, the amount of traffic of the requested packets is the traffic amount being generated by the aforementioned user terminals.
The second step: the reception quality of the shared channel at the position of each user terminal is estimated. To explain in detail, for each user terminal, the interference wave signal power of the packets having been received from multiple base stations is calculated repeatedly, so as to estimate and calculate the reception quality of HS-PDSCH (High Speed-Physical Downlink Shared CHannel) as the shared channel based on the calculated result.
The third step: based on the amount of traffic generated by each user terminal, the reception quality of the shared channel at the position of each user terminal and the packet scheduler at the base station, packets are transmitted from the base station to each user terminal, using the shared channel.
The fourth step: based on the number of bits of the packets having been received from the base station, user throughput is calculated for each user terminal. Once the fourth step is completed, the operation returns to the first step. At this moment, the position of each user terminal is updated in accordance with the speed at which the user terminal moves, and once again the first to fourth steps are repeated.
The fifth step: user throughputs as the calculated result at the fourth step are averaged. Then, the position of each user terminal is output and the average value of the user throughputs of all the user terminals are output as a user throughput estimation.
System level simulator 300 executes the first to fourth steps every 2 ms in real time. System level simulator 300 also performs simulation of the first to fourth steps every 2 ms for periods of one hour or longer in order to obtain statistically reliably user throughput estimation.
As a result of this, accuracy of user throughput estimation is improved. On the contrary, however, an enormous amount of processing is needed for simulation of the first to fourth steps every 2 ms for periods of one hour or longer, so the time for estimating user throughput becomes longer.
Further, since system level simulator 300 executes a process of averaging user throughputs at the fifth step in order to display the geographical distribution of user throughput, the time for estimating user throughput becomes further longer.
Taking a long time for execution of estimating user throughput gives rise to the following problem especially when the proper values for the base station configuration are examined. When the proper values for the base station configuration are deliberated, a system level simulation needs to be effected every time the base station configuration is altered. Accordingly, long-time system level simulation should be repeated many times until the proper values for the base station configuration is determined, so that the time for deliberation becomes extremely long.
Also, there are other conventional methods for estimating throughput: a method of determining the propriety of starting communication by estimating throughput (e.g., see Japanese Patent Application Laid-open 224094/2000), a method of improving system throughput (e.g., see Japanese Patent Application Laid-open 298498/2003), a method of realizing a base station capable of estimating packet quality (e.g., see Japanese Patent Application Laid-open 112597/2004) and the like.